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Posterior Fossa Tumors

The posterior fossa is a region near the base of the skull. Tumors that arise in this depression are of special concern because the posterior fossa is a small enclosed space near critical brain structures, including the brain stem, the cerebellum, and cranial nerves, and the tumors often can be difficult to treat. Most pediatric brain tumors, 55 to 70 percent, arise in the posterior fossa, compared with 15 to 20 percent of adult tumors.

The tumors that occur in the posterior fossa are more likely to be primary brain tumors, meaning that they arise from the tissues of the brain, rather than metastatic tumors, which arise in a different part of the body and spread to the brain. Posterior fossa tumors may be almost any type of primary brain tumor, including gliomas, astrocytomas, or hemangioblastomas, tumors that arise from glial cells, the supportive cells of the brain; ependymomas, tumors of the lining of the cavities in the brain; acoustic neuromas, benign tumors that grow from the sheath of a cranial nerve; meningiomas, tumors of the protective cover of the brain; medulloblastomas, malignant tumors that arise from incompletely developed cells; pineoblastomas, malignant tumors of the pineal gland; or other types of primary brain tumor.

Symptoms

Because of the location of posterior fossa tumors, they often grow to block the flow of cerebrospinal fluid, the fluid that bathes the brain and spinal cord, causing hydrocephalus, an increase in pressure inside the skull. Increased cranial pressure causes headaches, nausea, and vomiting. The posterior fossa is a small space surrounded by important functional structures. As the tumor grows to fill the space, it will begin to exert pressure on these structures, causing a range of other symptoms, including disturbances in vision and hearing, weakness or numbness in the face, and problems with balance and walking.

Diagnosis

Imaging studies are the key component in the diagnosis of posterior fossa tumors. Magnetic resonance imaging (MRI) is the best available imaging modality for these tumors. For this study, an agent that provides contrast in the image is administered intravenously so neurological surgeons can visualize the tumor against the normal brain in the background. In some cases, neurological surgeons may employ an MRI scan with frameless stereotactic guidance. For this study, a contrast MRI is performed after special markers (called fiducials) are placed on the patient's scalp. The fiducials are processed by a computer, which calculates the location of the tumor and creates a three-dimensional reconstruction. This image then is used at the time of surgery to help locate the tumor precisely, maximize tumor removal, and minimize injury to the surrounding brain.

Treatment

Surgery usually is the first treatment for tumors of the posterior fossa. Because of the proximity of these tumors to critical brain structures, even benign ones will be removed. The goal of surgery is to remove the entire mass to alleviate the pressure exerted on surrounding tissue. In addition, surgery is done to retrieve a biopsy to provide an accurate tumor diagnosis. Complete resection may be difficult because of the critical structures in the area that must be preserved. Depending on the patient and the extent of the resection, surgery may be followed by radiation therapy. In addition, some smaller tumors may be treated effectively with stereotactic radiosurgery, which involves the use of a highly focused beam of radiation to target the cancer cells specifically and leave the surrounding brain unaffected.

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